The use of a stylus with touchscreens is well known, but existing technologies can have the disadvantages of cost, performance and/or reliability. Resistive touchscreens can be well suited for use with a passive (i.e., non-powered) stylus. The PalmPilot personal digital assistant (PDA), launched in 1997, was one of the first devices with a resistive touchscreen designed for use with a stylus, and helped to popularize that technology. However, resistive touchscreens have many disadvantages, and are increasingly being replaced by capacitive touchscreens.
Capacitive touchscreens can support the use of a passive stylus, but in many cases, can require a minimum stylus tip size (e.g., 5 mm). Such a size can be much larger than a pen-like tip (e.g., 1 mm) desired in many applications.
Various tethered active (i.e., powered) stylus approaches have been deployed for use with capacitive touchscreens, and have been included in applications such as point-of-sale terminals (e.g., the signature pad used for credit card transactions in larger retail stores) and other public uses. However, the need for a cable (i.e., that tethers the stylus to a host device) can be a significant drawback for “private” applications such as tablets, personal computers (PCs), and smartphones.
Conventional technologies used in tethered applications can fall broadly into two categories: inductive and electrostatic. In inductive technologies, stylus sensing is implemented largely independently of the finger-sensing capability of the touchscreen. Typically, an AC signal is generated and fed to the tip of the stylus, and sensors behind or around the display receive the signal. A relative magnitude of the received signal at each of the sensors can then be used to interpolate the position of the stylus tip. In electrostatic technologies, an electrostatic field is generated at the tip of the stylus which is detectable by a self-capacitance touchscreen, as if the stylus tip was larger than it actually is. In effect, the electrostatic field is used to magnify the effective size of the stylus tip as detected by the self-capacitance sensing touchscreen system.
In order to meet the performance requirements demanded by many recent latest applications, touchscreens are rapidly migrating to mutual capacitance sensing—or a combination of self and mutual capacitance sensing.
Some conventional tether-free styluses have used a magnetic antenna for the synchronization signal for a host-to-stylus transmission. Synchronization is done by using a 13.56 MHz amplitude shift keying (ASK) signal. Two antennas are used. A transmitter antenna is implemented as 1÷3 turns coil, embedded in an indium tin oxide (ITO) portion of a touch screen panel routed at the host side. A receiver antenna is a coil placed inside the stylus. A drawback to such a conventional approach can be the high cost and complicated mechanical construction of the transmitter antenna.
Another type of stylus is a self-synchronized active stylus (SSAS). An SSAS can generate a square wave signal on a stylus tip. The signal waveform phase and frequency are not synchronized with the host. A host touch screen controller can receive the square wave signal and calculate a stylus touch position based upon the signal. One conventional SSAS system will now be described with reference to FIGS. 21A and 21B.
FIG. 21A shows a host end of a system 2100. A sense (ITO) panel 2101 can be connected to a selective receiver 2103. A sense panel 2101 can include a number of receive lines. Selective receiver 2103 can detect the panel receive line(s) with the maximum received signal to determine a stylus position. A detected stylus signal can also include force data which can be decoded by decoder 2105. A host CPU 2107 can provide force and position (Touch Signal) data for an application run by the host device 2109.
FIG. 21B is a block diagram of a conventional SSAS 2111. Conventional SSAS 2111 is an “active” stylus, and thus generates a signal for transmission to sense panel. A reference clock 2119 can generate signal that can be amplified by amplifier 2121 and driven on a stylus tip 2123, and thereby supplied to a host sense (i.e., ITO) panel. A stylus 2111 can also include a force sensor 2113 to detect the force at the stylus tip 2123 that is measured by measurer 2115. Force data are transmitted by modulating the carrier frequency with a modulator 2117. The signal transmitted by the stylus 2111 can induce a signal in the sense panel 2101 of the host system 2100, for subsequent decoding. Stylus 2111 can be powered by from a battery.
A drawback to a conventional SSAS like that of FIG. 21B can be limited life due to power consumption.